I spent a year trying all of the recommended AVs, firewalls, registry cleaners, HIPS programs, optimizers, defraggers and anti-spyware on four different XP-based PCs. (And to let you know I download stuff off of the Internet frequently, so I want the best.) Here's what I use:
AV: either ESET's NOD32 or Avira's Antivir - the paid versions. I haven't had any infections since using them. They both have rootkit detection. They both have pretty clunky interfaces...but their ease of use runs rings around Kaspersky unless you just use it "out of the box" - in which case it's not very effective. Antivir occasionally throws a false positive at me...but no big deal.
Firewall: COMODO - best there is, easy to use, has a VERY good HIPS function and it's free. Need I say more?
Anti-spyware - AVG (paid). They've discontinued it as a stand-alone...but I disregarded their "upgrade now" pitches and guess what... my program now says I have an unlimited license, and I continue to get signature updates; Spybot - everyone says this is worthless, but I like it...if for no other reason than its immunization feature; and Ad-aware 2008 (free) - it seems to catch everything the other two miss.
HIPS - I'm still undecided on this...PREVX 2 is my favorite so far...but I haven't bought it yet. I run PREVX CSI (a baby not-real-time version of PREVX2) as a double-check - since they have an extensiveonline database of bogeys from their user community.
Cleaners (Registry and file) - The best there is are two products from Superwin Software (ever heard of them? probably not) - A1 Click Ultra PC Cleaner and Regvac. I use CCleaner when I'm feeling lazy and don't have any problems. Regvac has saved me from doing a system rebuild more than once. You have to be a little careful with A1 not to clean too much!
Edit
Registry Cleaners, not recommended
Optimizers - Pick from several - each of them seem to have their own particular blend of what they consider optimized. I like Iobit's Advanced WindowsCare V2 Pro for all of the added tools and ease of use. But my all time favorite is Iolo's System Mechanic...I don't recommend their latest version because it is paranoid that you might not have a real license...stick with version 7 for now.
Defraggers - Perfectdisk 2008 - If you have a HD bigger than 80gigs, get this...period. Their super optimization function keeps your files from getting defragged in the first place. Other mentionables...Vopt 9 - very nice features including a cleaner that you can run before you defrag and a kill process function to speed up defrag time...and Iolo's SmartDefrag...it's free and good...has a background feature that helps keeps your files contiguous.
"And that's all I have to say about that...."